Peakforce photothermal-based detection of IR nanoabsorption

ABSTRACT

An apparatus and method of performing photothermal chemical nanoidentification of a sample includes positioning a tip of a probe at a region of interest of the sample, with the tip-sample separation being less than about 10 nm. Then, IR electromagnetic energy having a selected frequency, ω, is directed towards the tip. Using PFT mode AFM operation, absorption of the energy at the region of interest is identified. Calorimetry may also be performed with the photothermal PFT system.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. Pat. No. 9,719,916,issued Aug. 1, 2017 (U.S. application Ser. No. 15/256,071, filed Sep. 2,2016), which is a continuation of U.S. application Ser. No. 14/962,971,filed Dec. 8, 2015, which is a continuation of U.S. Pat. No. 9,207,167,issued Dec. 8, 2015 (U.S. application Ser. No. 14/618,863, filed Feb.10, 2015), which is a continuation of U.S. Pat. No. 8,955,161, issuedFeb. 10, 2015 (U.S. application Ser. No. 14/217,099, filed Mar. 17,2014), which claims priority from provisional application U.S. App. No.61/802,094, filed on Mar. 15, 2013, each of which is entitled Peak ForcePhotothermal-based Detection of IR Nanoabsorption. U.S. Pat. No.8,955,161 is also a continuation-in-part of U.S. application Ser. No.13/306,867 (U.S. Pat. No. 8,650,660), filed Nov. 29, 2011, which is anon-provisional of U.S. App. No. 61/417,837, filed Nov. 29, 2010, bothentitled Method and Apparatus of Using Peak Force Tapping Mode toMeasure Physical Properties of a Sample, which in turn is acontinuation-in-part of U.S. application Ser. No. 12/618,641 (U.S. Pat.No. 8,739,309), filed Nov. 13, 2009, which is a non-provisional of U.S.App. No. 61/114,399, filed Nov. 13, 2008, both entitled Method andApparatus of Operating a Scanning Probe Microscope.) The subject matterof these applications is hereby incorporated by reference in theirentirety.

BACKGROUND OF THE INVENTION Field of the Invention

The preferred embodiments are directed to making nano-identificationsample property measurements, and more particularly, using Peak ForceTapping® mode AFM and IR electromagnetic excitation for localizedphotothermal nanoID imaging and spectroscopy.

Description of Related Art

The interaction between a sample under test and radiated energy can bemonitored to yield information concerning the sample. In spectroscopy,dispersion of light from a sample into its component energies can bemeasured and, for example, intensity plotted as a function ofwavelength. By performing this dissection and analysis of the dispersedlight, users can determine the physical properties of the sample, suchas temperature, mass, and composition.

Notably, making spectroscopic measurements with a spatial resolution onthe nanoscale is continuing to improve. However, despite ongoingprogress in the development of imaging techniques with spatialresolution beyond the diffraction limit, simultaneous spectroscopicimplementations delivering chemical specificity and sensitivity on themolecular level have remained challenging. Far-field localizationtechniques can achieve spatial resolution down to about 20 nm bypoint-spread function reconstruction but typically rely on fluorescencefrom discrete molecular or quantum dot emitters, with limited chemicallyspecific information.

One technology for improving spectroscopic measurement performance isscanning probe microscopy. Scanning probe microscopes (SPMs), such asthe atomic force microscope (AFM), are devices which typically employ aprobe having a tip and causing the tip to interact with the surface of asample with appropriate forces to characterize the surface down toatomic dimensions. Generally, the probe is introduced to a surface of asample to detect changes in the characteristics of a sample. Byproviding relative scanning movement between the tip and the sample,surface characteristic data can be acquired over a particular region ofthe sample and a corresponding map of the sample can be generated.

A typical AFM system is shown schematically in FIG. 1. An AFM 10employing a probe device 12 including a probe 14 having a cantilever 15.Scanner 24 generates relative motion between the probe 14 and sample 22while the probe-sample interaction is measured. In this way images orother measurements of the sample can be obtained. Scanner 24 istypically comprised of one or more actuators that usually generatemotion in three orthogonal directions (XYZ). Often, scanner 24 is asingle integrated unit that includes one or more actuators to moveeither the sample or the probe in all three axes, for example, apiezoelectric tube actuator. Alternatively, the scanner may be anassembly of multiple separate actuators. Some AFMs separate the scannerinto multiple components, for example an XY scanner that moves thesample and a separate Z-actuator that moves the probe. The instrument isthus capable of creating relative motion between the probe and thesample while measuring the topography or some other surface property ofthe sample as described, e.g., in Hansma et al. U.S. Pat. No. RE 34,489;Elings et al. U.S. Pat. No. 5,266,801; and Elings et al. U.S. Pat. No.5,412,980.

In a common configuration, probe 14 is often coupled to an oscillatingactuator or drive 16 that is used to drive probe 14 at or near aresonant frequency of cantilever 15. Alternative arrangements measurethe deflection, torsion, or other motion of cantilever 15. Probe 14 isoften a microfabricated cantilever with an integrated tip 17.

Commonly, an electronic signal is applied from an AC signal source 18under control of an SPM controller 20 to cause actuator 16 (oralternatively scanner 24) to drive the probe 14 to oscillate. Theprobe-sample interaction is typically controlled via feedback bycontroller 20. Notably, the actuator 16 may be coupled to the scanner 24and probe 14 but may be formed integrally with the cantilever 15 ofprobe 14 as part of a self-actuated cantilever/probe.

Often a selected probe 14 is oscillated and brought into contact withsample 22 as sample characteristics are monitored by detecting changesin one or more characteristics of the oscillation of probe 14, asdescribed above. In this regard, a deflection detection apparatus 25 istypically employed to direct a beam towards the backside of probe 14,the beam then being reflected towards a detector 26. As the beamtranslates across detector 26, appropriate signals are transmitted tocontroller 20, which processes the signals to determine changes in theoscillation of probe 14. In general, controller 20 generates controlsignals to maintain a relative constant interaction between the tip andsample (or deflection of the lever 15), typically to maintain a setpointcharacteristic of the oscillation of probe 14. For example, controller20 is often used to maintain the oscillation amplitude at a setpointvalue, As, to insure a generally constant force between the tip andsample. Alternatively, a setpoint phase or frequency may be used.

A workstation 40 is also provided, in the controller 20 and/or in aseparate controller or system of connected or stand-alone controllers,that receives the collected data from the controller and manipulates thedata obtained during scanning to perform point selection, curve fitting,and distance determining operations.

AFMs may be designed to operate in a variety of modes, including contactmode and oscillating mode. Operation is accomplished by moving eitherthe sample or the probe assembly up and down relatively perpendicular tothe surface of the sample in response to a deflection of the cantileverof the probe assembly as it is scanned across the surface. Scanningtypically occurs in an “x-y” plane that is at least generally parallelto the surface of the sample, and the vertical movement occurs in the“z” direction that is perpendicular to the x-y plane. Note that manysamples have roughness, curvature and tilt that deviate from a flatplane, hence the use of the term “generally parallel.” In this way, thedata associated with this vertical motion can be stored and then used toconstruct an image of the sample surface corresponding to the samplecharacteristic being measured, e.g., surface topography. In one mode ofAFM operation, known as TappingMode™ AFM (TappingMode™ is a trademark ofthe present assignee), the tip is oscillated at or near a resonantfrequency of the associated cantilever of the probe. A feedback loopattempts to keep the amplitude of this oscillation constant to minimizethe “tracking force,” i.e. the force resulting from tip/sampleinteraction. Alternative feedback arrangements keep the phase oroscillation frequency constant. As in contact mode, these feedbacksignals are then collected, stored, and used as data to characterize thesample. Note that “SPM” and the acronyms for the specific types of SPMs,may be used herein to refer to either the microscope apparatus or theassociated technique, e.g., “atomic force microscopy.” In a recentimprovement on the ubiquitous TappingMode™, called Peak Force Tapping®(PFT) Mode, feedback is based on force as measured in each oscillationcycle.

Regardless of their mode of operation, AFMs can obtain resolution downto the atomic level on a wide variety of insulating or conductivesurfaces in air, liquid, or vacuum by using piezoelectric scanners,optical lever deflection detectors, and very small cantileversfabricated using photolithographic techniques. Because of theirresolution and versatility, AFMs are important measurement devices inmany diverse fields ranging from semiconductor manufacturing tobiological research.

Infrared (IR) spectroscopy is a useful tool in many analytical fieldssuch as polymer science and biology. Conventional IR spectroscopy andmicroscopy, however, have resolution on the scale of many microns,limited by optical diffraction. It has become apparent that it would beparticularly useful to perform IR spectroscopy on a highly localizedscale, on the order of biological organelles or smaller, at variouspoints on a sample surface. That way, information about the compositionof the sample, such as location of different materials or molecularstructures, would be possible.

Conventional infrared (IR) spectroscopy is a widely used technique tomeasure the characteristics of material. In many cases, the uniquesignatures of IR spectra can be used to identify unknown material. IRspectroscopy is performed on bulk samples which gives compositionalinformation but not structural information. Infrared spectroscopy allowscollection of IR spectra with resolution on the scale of many microns.Near-field scanning optical microscopy (NSOM) has been applied to somedegree in infrared spectroscopy and imaging. While there have been someadvancement with NSOM, the field is still in need of a sensitive andreliable commercial instrument. At this time, no widely availableinstrument provides routine IR spectra with resolution below thediffraction limit. One technique based on use of an AFM to produce suchlocalized spectra is described in a publication entitled “Local InfraredMicrospectroscopy with Sub-wavelength Spatial Resolution with an AtomicForce Microscope Tip Used as a Photo-thermal Sensor” (PTIR) OpticsLetters, Vo. 30, No. 18, Sep. 5, 2005. The technique is also discussedin US Pub. No. 2009/0249521 (The '521 publication), the entirety ofwhich is expressly incorporated by reference herein. Those skilled inthe art will comprehend the details of the technique in the publicationbut the technique will be described briefly herein for clarity.

Referring to the '521 publication, in PTIR, infrared radiation isincident on a region of a sample. At a wavelength absorbed by thesample, the absorption will typically cause a local increase intemperature and a rapid thermal expansion of the sample. A probe isarranged to interact with the sample and transducer to generate a signalrelated to the IR energy in the region under the probe tip. “Interact”means positioning the probe tip close enough to the sample such that aprobe response can be detected in response to absorption of IRradiation. For example, the interaction can be contact mode, tappingmode or non-contact mode. An associated detector can be used to read oneor more probe responses to the absorbed radiation. The induced proberesponse may be a probe deflection, a resonant oscillation of the probe,and/or a thermal response of the probe (e.g., temperature change). Forprobe deflection and/or resonant oscillation of the probe, appropriatedetectors can include split segment photodiodes along with anyassociated amplification and signal conditioning electronics. In thecase of a thermal response, the appropriate detector may comprise, forexample, a Wheatstone bridge, a current and/or voltage amplifier and/orother associated electronics to sense, amplify, and condition thethermal signal from the probe. The probe response is then measured as afunction of the wavelength of incident radiation to create an absorptionspectrum. From the spectra, material in the sample can be characterizedand/or identified.

As noted in the '521 publication, an AFM set-up used for the publishedwork on IR spectroscopy is shown. The sample is mounted on a ZnSe prism,or prism made from other suitable materials, which does not absorb theradiation of interest. A pulsed IR source, in this case a Free ElectronLaser beam, is directed into the prism. The prism is made at an anglesuch that the beam is in Total Internal Reflection in order for the beamto be propagative in the sample and evanescent in the air. Thus, onlythe sample is significantly exposed to the laser radiation, and the AFMprobe is minimally exposed to the beam. The Free Electron Laser (FEL) isan IR source that is both variable in wavelength and has a pulsedoutput. Free Electron Lasers are large expensive facilities. The probeis placed at a point on the sample by the scanner and is held at anaverage height by feedback electronics. Both the vertical and lateraldeflection signal, as well as the feedback signal, may be monitored.

When the FEL is pulsed, the sample may absorb some of the energy,resulting in a fast thermal expansion of the sample as shown in FIG. 3.This has the effect of a quick shock to the cantilever arm, which, ifthe ability of the cantilever to respond to this shock is slower thanthe shock, will result in exciting a resonant oscillation in thecantilever arm. Because the absorbed energy is ideally contained withinthe sample, this shock is due primarily to rapid sample expansion, asminimal IR energy is absorbed by the cantilever itself. Although theprobe is kept in contact with the surface by the feedback electronics,the resonant signal is too fast for the feedback electronics, but can beobserved directly from the photodetector. Thus the cantilever rings downwhile still in contact with the surface, an effect called “contactresonance”. The absolute deflection, amplitude, and frequencycharacteristics of the contact resonance vary with the amount ofabsorption, as well as other properties, such as the local hardness, ofthe localized area around the probe tip, for example, by analyzing theringdown and/or the Fourier transform (FFT) of the ringdown events.Also, depending on the direction of the expansion, vertical resonances,lateral resonances, or both can be excited. By repeating the aboveprocess at varying wavelengths of the FEL, an absorption spectra on alocalized scale is achieved. By scanning the probe to various points onthe sample surface and repeating the spectra measurement, a map of IRspectral surface characteristics can be made. Alternatively, thewavelength of the FEL can be fixed at a wavelength that ischaracteristic of absorption of one of the components of the sample. Theprobe can then be scanned across the sample surface and a map of thelocation of that component can be generated.

Although the set-up as described is promising, there is no realpossibility of commercializing the same. First, the IR light sourceused, the Free Electron Laser, is a very large and expensive facility,as noted. Moreover, alternative benchtop sources of IR radiation havebeen limited by one or more characteristics that have made themunsuitable for a widely available instrument.

As further noted in the '521 publication, the apparatus described in theliterature suffers from other limitations beyond the expensive andstationary IR source. The apparatus employs a bottoms-up illuminationscheme that requires a sample to be placed on a specially fabricated IRtransmitting prism. In addition to being costly and easy to damage, thisarrangement requires special sample preparation techniques to prepare asample thin enough such that the IR light can penetrate the sample toreach the probe. Further, the actual signals generated can be small,thus requiring averaging of the signal and limiting the bandwidth of thetechnique. More sensitivity is required to address a wider range ofpotential samples.

The applicant associated with the '521 publication contends that thesystem disclosed therein can be used to obtain IR spectra from highlylocalized regions of a sample, allowing discrimination and/oridentification of the composition of a micro or nano-sized region of asample. The system may be used for mapping the variations in IRabsorption over a wider area of a sample, by imaging the energy absorbedat one or more wavelengths. One additional problem, however, is leakageof heat from a region that actually is absorbing the excitation energy,to a region that is not. When using PTIR, as the sample is heated (i.e.,a region of interest exhibits absorbing characteristics), the heat may,and often does, leak toward surrounding regions of the sample. If theprobe scans a location that is indirectly heated, the instrument mayidentify that location as being responsive to the IR excitation andconclude it is absorbing, when in fact it was not. Clearly, this canlead to compromised data and/or poor resolution. Independent of itsability to provide localized spectroscopy, an improved IR microscopyinstrument that can provide efficient localized spectroscopicmeasurements was desired.

Despite ongoing progress in the development of imaging techniques withspatial resolution beyond the diffraction limit, spectroscopicimplementations delivering chemical specificity and sensitivity on themolecular level have remained challenging. Again, far-field localizationtechniques can achieve spatial resolution down to about 20 nm bypoint-spread function reconstruction but typically rely on fluorescencefrom discrete molecular or quantum dot emitters, with limited chemicallyspecific information. Scanning near-field optical microscopy (SNOM)provides sub-diffraction-limited resolution through the use of taperedfibers or hollow waveguide tips. However, aperture-limited andwavelength-dependent fiber throughput reduces sensitivity, generallymaking SNOM unsuitable for spectroscopic techniques that have lowintrinsic signal levels.

In scattering-type SNOM (s-SNOM) external illumination of a sharp(metallic or semi-conducting) probe tip can enhance sensitivity,spectral range, and spatial resolution. Chemical specificity can beobtained through the implementation of, for example, IR vibrationals-SNOM, tip-enhanced coherent anti-Stokes Raman spectroscopy (CARS), ortip-enhanced Raman scattering (TERS). Here the antenna or plasmonresonances of the (noble) metal tips can provide the necessary fieldenhancement for even single-molecule sensitivity.

In the standard implementation, however, the direct illumination of thetip apex results in a three-to-four orders of magnitude loss inexcitation efficiency, related to the mode mismatch between thediffraction-limited far-field excitation focus and the desired tens ofnanometers near-field localization, as determined by the tip apexradius. The resulting loss of sensitivity, together with a far-fieldbackground signal, often limit contrast and may cause imaging artifacts,presenting challenges for the general implementation of a wider range ofspectroscopic techniques in s-SNOM.

Given the interest in spectroscopy-related characteristics of samples ona much smaller scale, an improved instrument was desired to expand therange and efficiency of performing optical imaging and spectroscopy forchemical identification on the nanoscale.

SUMMARY OF THE INVENTION

Using Peak Force Tapping® mode AFM, the preferred embodiments overcomethe drawbacks with the prior art, including PTIR, by directing lightoverhead of the sample and locally exciting the photothermal response atthe tip-sample interface. Resolution is improved and sample preparationis minimized.

In one preferred embodiment, a method of locally measuring IR absorptionof a sample, the method includes causing a probe to interact with asample in an oscillating mode of AFM operation, and then directing alocally amplified IR signal at a sample. The method identifies a changein modulus based on the directing step to provide an indicator of IRabsorption by the sample. Measuring techniques sensitive to moduluschange, such as peak force tapping (PFT) AFM mode, or contact resonancemode, may be employed.

In another preferred embodiment, a method of photothermal chemicalnanoidentification of a sample includes positioning a tip of a probe ata region of interest of the sample, the tip-sample separation being lessthan about 10 nm. Then IR electromagnetic energy having a selectedfrequency, ω, is directed towards the tip. Using PFT mode AFM operation,absorption of the energy at the region of interest is identified.

In another aspect of the preferred embodiments, the method furtherincludes obtaining a spectrum of the sample by tuning the selectedfrequency to a range of frequencies.

According to a further aspect of the preferred embodiments, a method ofphoto thermal nanocalorimetry on a sample using an AFM operating in PeakForce Tapping® mode includes positioning a tip at a region of interestof the sample. The method then directs IR electromagnetic energy havinga selected frequency, ω, towards the tip, and determines a mass of theregion by obtaining 3D topography data corresponding to the region inresponse to the directing step. A ΔT in response to the directing stepis also determined to ultimately provide an indication of heat capacity.

These and other features and advantages of the invention will becomeapparent to those skilled in the art from the following detaileddescription and the accompanying drawings. It should be understood,however, that the detailed description and specific examples, whileindicating preferred embodiments of the present invention, are given byway of illustration and not of limitation. Many changes andmodifications may be made within the scope of the present inventionwithout departing from the spirit thereof, and the invention includesall such modifications.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred exemplary embodiments of the invention are illustrated in theaccompanying drawings in which like reference numerals represent likeparts throughout, and in which:

FIG. 1 is a schematic illustration of a Prior Art atomic forcemicroscope AFM;

FIG. 2 is a schematic illustration of a PeakForceIR experimental set-upaccording to a preferred embodiment, including an AFM probe tip excitedby an IR source and interacting with a sample;

FIG. 3 is a schematic front view of a PFT probe of the PeakForceIRapparatus, illustrating localized heating of a sample region ofinterest;

FIG. 4 is a flow diagram of a nanoidentification method usingPeakForceIR, according to a preferred embodiment;

FIG. 5 is a graph of Young's modulus vs. temperature for a range ofmaterials; and

FIG. 6 is a flow diagram of a PeakForceIR method of performingnanocalorimetry, according to a preferred embodiment

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An improved apparatus and method of performing chemical identificationof a sample includes combining monochromatic IR excitation at a selectedfrequency with Peak Force Tapping® (PFT) AFM configured for makingmechanical property measurements. Purely mechanical detection of IRnanoabsorption by a sample is realized, thereby facilitating chemicalnanoidentification of a region of the sample.

Nanoscale Chemical Identification

Turning initially to FIG. 2, a PeakForceIR apparatus 80 includes a PFTprobe 82 that is introduced to a sample 84, and in particular, a regionof interest 86. A source of IR electromagnetic radiation 88 directslight toward a tip 90 of probe 82, tip 90 including an apex of nanoscaledimensions 92 that primarily interacts with the surface of sample 84.

FIG. 3 illustrates more specifically the localized heating of the sampleprovided by the IR source. Source 88 directs IR light at a selectedfrequency, ω, toward sample 84 with an optical element (e.g. a lens) 93focusing the light on tip 92 which through the lightning rod effect oran optical resonance (e.g. plasmonic) creates an enhanced, localizedelectric field 94 in the nanoscale gap between tip apex 92 and thesample 84 surface region of interest. If the material directlyunderneath the tip apex 92 is able to absorb the monochromatic IR lightat the IR frequency, i.e. it has a nonzero imaginary index of refractionat said frequency: Imag(n(w))>0, such as in the case of a molecularresonance, then a temperature rise will occur due to the absorption ofenergy from the IR light by the sample region. Essentially, a nanoscale“hot spot” under the tip is created. This temperature rise may cause ameasurable change in a mechanical property, i.e. elastic modulus, of thesample region, detectable by mechanical property measurement techniquessuch as PFT mode AFM.

By tuning the IR frequency (for example, sweeping), the user can obtaina spectrum of a sample region's change in mechanical properties as afunction of IR frequency. The largest mechanical property change willoccur for the largest temperature change, which in turn takes place atIR frequencies corresponding at or near the absorption maximum of thesample, specifically at the maximum of Imag(r_(p)(k,w)), where r_(p)(w)is the in-plane momentum (k) dependent, p-polarized reflectioncoefficient of the sample surface. The in-plane momentum is largelydetermined by the tip radius, R, and spans a range on the order of k=0to ˜3/R. Thus a plot of the IR laser induced mechanical property change(e.g. elastic modulus Y measured with and without the IR laserilluminating the tip) vs. IR laser frequency, provides a spectrumrelated to Imag(r_(p)(k,w)), which in turn is related to the sampleregion's absorption: Imag(n(w)). Locations where the largest change inmechanical properties takes place, is typically indicative of thelargest temperature change and thus the largest absorption. This willprovide an indication of the materials identity, since absorption linesare routinely used for chemical identification in infrared spectroscopy.If absorption does not occur at a particular (x, y) location, the lightwill simply be scattered and ignored in this system. Since themechanical property measurement in the PFT AFM mode is localized to thenanoscale, the IR laser induced change in mechanical properties detectedby such means would enable a nanoscale chemical identification modality.

A method 100 associated with this technique is illustrated in FIG. 4.Once a sample is loaded in the instrument in Block 102, the tip-sampleseparation (“s”) is reduced or periodically reduced to a positionsufficient to create the localized enhanced electric field between thetip apex and the sample in Block 104. Then, in Block 106 the tip isilluminated by the IR monochromatic source operated at a frequencyselected corresponding to an absorption line or other spectral featurefor which Imag(n(w))>0 for the material in the sample the user isseeking to identify, which will create an enhanced localized field inthe tip-sample gap. With the tip of the probe positioned above theregion containing the material of interest, PFT mode AFM is preferablyemployed to detect changes in the mechanical properties, e.g., theelastic modulus of the sample, if any, in Block 108. Measurements arerecorded in Block 110 and method 100 returns to Block 106 for the casein which the user would like to try to identify another material in thesample region of interest (by selecting an appropriate frequency of theIR source). As a preferred embodiment, the elastic modulus change due tothe IR laser excitation is detected. An illustration of a typical plotof Young's modulus for several different materials is provided in FIG.5. As noted, a change in temperature, ΔT of about 10 Kelvin causes a ΔY(change in Young's modulus) greater than 5% for common polymers such aspolystyrene and polyvinyl chloride. As can be calculated based on themodel provided in the previously mentioned PTIR publication, atemperature rise of the sample region beneath the tip on the order of10K is achievable on a sufficiently strong IR resonance using modestpower (<100 mW max power) pulsed or CW laser sources. Such a changeproduces a relative modulus change within the current detectionsensitivity of PFT. The detection sensitivity of PFT is described in thepreviously identified application incorporated by reference hereinnamely, U.S. Ser. No. 13/306,867, filed Nov. 29, 2011, entitled Methodand Apparatus of Using Peak Force Tapping Mode to Measure PhysicalProperties of a Sample, and demonstrated in Pittenger et al., BrukerNano Surfaces Division Application Note #128: Quantitative MechanicalProperty mapping at the Nanoscale with PeakForce QNM (Apr. 15, 2010),which is expressly incorporated by reference herein. By using PFT, oranother modulus sensitive measurement (e.g., contact resonance, in somecases, TappingMode, etc.), to record IR frequency dependent inducedsignals related to the Young's modulus, a spectrum of ΔY(w) can beobtained that will be directly comparable with an FTIR absorptionspectrum.

In one example, in FIG. 20B of the aforementioned '867 case, theinteraction force as a function of tip-sample distance in one modulationperiod is shown. The elastic property of the material can be calculatedconventionally by using the upper part of the slope (see segment DE inFIG. 20B, segments CDE illustrate short range repulsive interaction)using, for example, the Oliver-Pharr model, or another contactmechanical model. (see, e.g., Oliver W C and Pharr G M 2004 Measurementof Hardness and Elastic Modulus by Instrumented Indentation: Advances inUnderstanding and Refinements to Methodology J. Mater. Res. 19 Mar. 20,2004). A benefit of the peak force tapping (PFT) control is the abilityto use cantilevers from 0.01 N/m to 1000 N/m in one mode of AFMoperation, thereby enabling high resolution mechanical property mappingof the broadest range of materials on a single instrument from 10 kPa to100 GPa in elastic modulus.

In this way, direct measurement of absorption is provided (in contrastto the complex valued sSNOM near-field signal). Moreover, unlike PTIR,with the incident electromagnetic energy corning from the top of thesample (as opposed to underneath), sample selection is not limited(transparent materials, etc.), and the sample does not need to beprepared (complex slicing operation of the sample to several hundrednanometers). Overall, PeakForce IR provides an elegant solution that iseasier to set-up and operate, and more cost-effective than other purelyoptical chemical nanoidentification methods of such as sSNOM and TERS.

Nanoscale Calorimetry with PeakForceIR

A second application using a set-up similar to that described above forPeakForceIR is nanoscale calorimetry—heat capacity. The nanoscaletemperature rise of a quantifiable volume given a known input of energy,allows the user to determine nanoscale heat capacity of thermostaticmaterials. The heat capacity is given by,[C]=Q/mΔT  (Equation 1)in which m is the mass, Q is the heat input and ΔT is the temperaturerise. The mass is preferably provided by modeling via 3D topography,while Q is modeled from the laser power, or measured using sSNOM. Thetemperature rise, ΔT, is preferably measured using PeakForce mode formechanical property measurement (PeakForce QNM, the subject of theapplication expressly incorporated by reference herein).

A method 150 of nanoscale calorimetry is provided in FIG. 6. Uponloading a sample upon start-up and initialization in Block 152,electromagnetic energy is input to the sample via the PFT AFM tip inBlock 154. The corresponding temperature rise is measured usingPeakForceQNM mechanical property mapping in Block 156. Thereafter inBlock 158, the mass is determined by using a 3D topography image of theregion of interest. The heat capacity is then calculated (see, e.g.,Eqn. 1) in Block 160 and the process is repeated for differentexcitation frequencies and sample locations.

IR Source

The preferred embodiments use a rapidly tunable, electronicallycontrolled, monochromatic source of said radiation with a line widthbelow 1 cm⁻¹ and a peak power of at least 1 mW, capable of operating inpulsed mode. This enables imaging at a single, well-defined frequency,while still retaining the ability to perform spectroscopy (generate aspectrum) by rapidly tuning the laser and repeating the measurement ofthe near field interaction. One preferred implementation of such amonochromatic source is a tunable External Cavity Quantum Cascade Laser(QCL), which is routinely capable of producing an average spectral powerdensity of greater than 10 mW/cm⁻¹ with tuning ranges spanning >100cm⁻¹.

As best understood, such a spectral density is at least a factor of 1000better than other state of the art methods for producing broadband,coherent laser radiation over a bandwidth of 100 cm⁻¹, using methodssuch as difference frequency generation. The advantage in spectral powerdensity enables the acquisition of near field signals with sufficientsignal-to-noise performance for a 20×20 nm sample area on asub-millisecond timescale. Such an acquisition rate is essential todisplay a near field image in real-time, simultaneously with an AFMimage, for which a scan rate of 1 Hz or greater is preferred.Furthermore, when performing nanoID, oftentimes the shape of the entirespectrum is not necessary to successfully ID a sample region. Theabsorption at several well-chosen frequencies is oftentimes sufficient.Thus the ability to have the spectral density of a monochromatic laserat a select set of frequencies would allow for a far more rapid means ofnanoidentifying a material by its chemical signature. The ability torapidly tune the laser by as little as 0.01 cm⁻¹, allows a spectroscopicresolution difficult, if not impossible, to achieve with the broadbandapproach. Also, the user can specify an arbitrary set of frequencies ofinterest, which may correspond to several absorption lines, and acquirenear-field signals only at those frequencies.

Secondly, the ability for the monochromatic source to operate in pulsedmode is also preferred given the following advantages, such as:

-   -   1. the ability to trigger pulses at specific times during which        a near field interaction is at a desired state, i.e. a maximum        or a minimum (see discussion of background        subtraction/suppression below).    -   2. the ability to combine such laser pulses with the distance        correlated signal discriminator to z-gate the near-field        nanoantenna-sample surface interaction.    -   3. the ability to reach larger peak power during the time of the        pulse (elicits a stronger near-field response—easier to detect,        etc.).    -   4. the ability to perform a time-resolved near-field        measurement.    -   5. less heat generation which has the potential to allow        enhanced performance such as better stability and broader        tenability.    -   6. less heat generation at the nanoantenna-sample interface,        which reduces thermally induced vibrational noise in the        nanoantenna structure, and also produces a less destructive        measurement since the temperature of the nanoantenna and sample        are increased a lesser amount than in the case of a CW        excitation.    -   7. Typically lower cost due to a decreased design complexity and        lower cooling requirements over CW (continuous wave) lasers.        Probe Design

The optimal probe for concentrating incident IR fields at the probe apexis one which can oscillate above the surface, has a tip radius below 50nm, is an excellent optical antenna in the IR range of interest, andprovides a high degree of side optical access to the probe. The qualityof the probe's optical antenna effect depends on the IR conductivity ofthe probe's material composition, to the extent of the skin depth at theIR frequency of interest. Higher IR conductivity for the top ˜10-100 nmof the probe material results in larger surface currents generated alongthe tip length in response to an incident IR field. Larger currentsresult in larger dynamic charge concentrations at the tip apex which inturn create larger localized near fields in the tip and sample gap. Asmaller tip radius also serves to increase the field enhancement betweenthe tip and sample. One embodiment of such a probe is a conventionalsilicon probe which has been metallized by a Platinum Iridium orPlatinum Silicide coating. Such probes can also be made forward facingwith a visible apex, which can enhance the side optical access to theprobe. If a coated tip is used, the coating should be durable so that itdoes not deteriorate during scanning, causing a loss of antenna quality.Alternatively, a solid metal probe can be employed or a probe ofsufficiently high carrier mobility, such as heavily doped Silicon orDiamond.

The optimal probe for nanoscale mechanical measurements of stiffness,such as those measurements performed using PeakForce QNM, requires aprobe with a stiffness in an optimal range for the expected stiffness ofthe material being investigated. For applications involving polymers, aprobe with a spring constant on the order of k>10 N/m and a tip radiusR<10 nm is sufficient. For other harder or softer material classesprobes with more appropriate spring constants may be optimal. The rangeof modulus values currently covered by available probes is approximately700 kPa-70 GPa, as described in the aforementioned incorporatedApplication Note regarding PeakForce QNM.

For the combined technique described with respect to this invention, aprobe that is both a highly efficient concentrator of IR light andpossesses a stiffness optimized for the material category underinvestigation, is highly desired. Such a probe needs to be an excellentoptical antenna with a high degree of optical access to the tip whilebeing much stiffer than the typical material investigated with the IRtechnique described here. For instance, a Silicon probe with a forwardfacing tip metallized with a ˜10 nm layer of Platinum Silicide orPlatinum Iridium attached to a cantilever having a spring constant of10-40 N/m is one possible choice which is already commerciallyavailable. Similarly, softer materials may be employed using a coatedprobe having lower spring constants, such as 0.5 N/m.

Although the best mode contemplated by the inventors of carrying out thepresent invention is disclosed above, practice of the above invention isnot limited thereto. It will be manifest that various additions,modifications and rearrangements of the features of the presentinvention may be made without deviating from the spirit and the scope ofthe underlying inventive concept.

What is claimed is:
 1. A method of photo thermal nanocalorimetry on asample using an AFM operating in Peak Force Tapping mode, the methodincluding: positioning a tip at a region of interest of the sample;directing IR electromagnetic energy having a selected frequency, ω,towards the tip; determining a mass of the region by obtaining 3Dtopography data corresponding to the region in response to the directingstep; and determining a ΔT in response to the directing step.
 2. Themethod of claim 1, wherein the tip has an apex with a radius betweenabout 10 nm and 100 nm.
 3. The method of claim 2, wherein the tip ismade so as to concentrate IR fields along a length of the tip inresponse to the directing step.
 4. The method of claim 3, wherein an IRconductivity of the distal 10 nm to 100 nm of the tip is higher than theremainder of the tip.
 5. The method of claim 4, wherein the tip issilicon and is metalized with a Platinum Iridium or Platinum Silicidecoating.
 6. The method of claim 3, wherein the tip is one of a solidmetal probe and a probe made of a heavily doped material.
 7. The methodof claim 1, further comprising calculating a heat capacity, [C], of theregion according to [C]=Q/mΔT.
 8. The method of claim 7, furthercomprising repeating each of the previous steps at a plurality ofregions of interest.
 9. The method of claim 7, further comprisingrepeating each of the previous steps at a plurality of selectedfrequencies, ω, at the regions of interest.
 10. The method of claim 1,wherein the directing step includes using an IR source that ismonochromatic with a line width below 1 cm⁻¹ and a peak power greaterthan 1 mW.
 11. The method of claim 10, wherein the monochromatic sourceis a tunable Quantum Cascade Laser (QCL).
 12. The method of claim 1,wherein ΔT is measured using Peak Force QNM mode.